This invention provides a new method for providing an undercoat for "particulate media". Viewed differently, this patent also teaches a new undercoating for particulate media, one which is in one sense similar to the undercoating for "thin-film" media. Particulate technology has been available for about 25 years and is well understood to be a reliable media. It is currently the predominate source of magnetic media which is typically found in computer storage devices.
In magnetic disk drive storage devices, the thinner the magnetic media coating, the smoother it is and the smoother it is, the closer the read/write heads may fly to it. However, where the undercoat (and the substrate) is not smooth, a very thin media coating can not be used since it will not cover the deformities and aberrations from flatness unless the coating is thick enough to fill the valleys and pits and cover the peaks and ridges. At the higher bit-densities required in high density disk drives, the read/write heads need to fly extremely close to the media on the disk in order to read the smaller magnetic bit transitions accurately. (Fly height for these heads is less than 10 microinches or less than 0.25 microns.) With this invention the cheaper particulate media coatings may be uaed to achieve bit-densities herefore only available on thin film media.
Unfortunately it is quite costly to sputter media onto disks (which is the process used to apply the thin film media coating). It requires a high energy magnetic field in a controllable vacuum chamber and uses expensive, carefully produced "targets" or sources of magnetic thin film coating.
Spin coating, on the other hand, is substantially cheaper and produces what is called a particulate media. (because of the particles of magnetic media (usually iron oxide) in the magnetic suspension when in liquid form), but, it produces it as a thin coating, somtimes called a thin film. To avoid confusion, all further reference in this patent to "thin film" refers to coated media and "particulate" refers to spin coated media. An example of spin coating is described in U.S. Pat. No. 4,551,355, issued to Ericson and Shadzi, and incorporated herein by reference.
Thin film media is generally a metal alloy (NiCoCr, for example). Particulate media in its pre-coated form is usually Iron-oxide or Barium-Ferrite or similar particles in a suspension containing a mixture of polymeric binders. When the solvent is evaporated and the polymeric binders react during curing, the magnetic particles get held in a positional orientation by binding matrix. The binding matrix is usually primarily attached by mechanical linkages to the Ni-P undercoat, although probably some chemical bonding is also occurring. The binding polymers may be of various formulations but a one to one ratio of phenolic and phenoxy resins may be used.
It is an important feature of the undercoat that it be non-magnetic For particulate media the traditional undercoating has been a Chromate-Phosphate conversion coating. However, Chromate-Phosphate solution reacts with the impurities in the Aluminum substrate, the end result of which is areas or pits at which the later coatings will either not stick or cause "drop-outs". Drop-outs are areas ot the disk which, when assembled into a disk drive, cannot be written to and read from. Chromate-Phosphate is also relatively expensive.
In this short, Ni-P undercoating for particulate media was unknown prior to this invention the particulars of which are described below.